1. Field of the Invention
This invention relates to a process for the purification of silicon. More particularly, this process relates to a process for purifying solid silicon which has been heated to a temperature slightly below the melting point of silicon while contacting the heated silicon with a purifying agent which does not appreciably react with the silicon.
2. Description of the Prior Art
An increasing demand for silicon of sufficiently high purity to be suitable for use in the semiconductor and solar cell industries has lead to investigation of many processes to achieve such purity levels. Such processes typically involve some sort of treatment of molten silicon. Purification of silicon in a molten state is, however, not new. For example, Allen U.S. Pat. No. 1,037,713 describes the purification of silicon by treating molten silicon with metals, such as alkali metals and alkaline earth metals including magnesium.
Brockbank U.S. Pat. No. 1,180,968 describes melting silicon under a slag of natural or artificial silica to eliminate impurities while Pacz U.S. Pat. No. 1,518,872 describes silicon as a valuable byproduct of a reaction between aluminum powder and a metallic fluorosilicate, such as magnesium fluorosilicate.
Pruvot et al U.S. Pat. No. 3,034,886 describes the purification of silicon or ferrosilicons by the injection of silicon fluoride gas into the liquid bath to react with aluminum and calcium impurities to form aluminum and calcium fluorides.
The use of molten metal fluorides for purification of silicon at a temperature of 1000.degree.-1600.degree. C. has been proposed by Coursier et al U.S. Pat. No. 3,148,131. The patentees, however, propose the use of metal fluorides which, in the main, either represent costly materials or materials known to react with silicon to form silicon fluoride and inject impurities in the silicon that are detrimental to its electronic properties.
Boulos U.S. Pat. No. 4,379,777 teaches passing powdered silicon through a plasma which apparently causes migration of the impurities to the surface of the molten silicon particles. After quenching, the particles are acid-leached to remove the surface impurities.
Kapur et al U.S. Pat. No. 4,388,286 combines vacuum refining of silicon with mixing the silicon with an effective fluxing agent, such as a fluoride of an alkali metal or an alkaline earth metal, to form a molten silicon phase and a slag phase.
One of us has also authored or coauthored papers which refer to the purification of molten silicon in contact with NaF in "Silicon Sheet for Solar Cells", by A. Sanjurjo published in the Journal of the Electrochemical Society, Volume 128, pp. 2244-2247 (1981) and "Fluxing Action of NaF on Oxidized Silicon", by L. Nanis, A. Sanjurjo, and S. Westphal published in Metallurgical Transactions B, Volume 12B, pp. 535-573 of the American Society for Metals and the Metallurgical Society of AIME (1981).
Not all prior purification processes, however, involve the melting of silicon. Ingle U.S. Pat. No. 4,172,883 discloses a process for purifying metallurgical grade silicon by heating it to 800 to 1350.degree. C. and contacting it with silicon fluoride gas which is said to react with the impurities causing them to deposit out. The aforementioned Coursier et al patent also speaks of purification temperatures below the melting point of silicon.
It is also known to purify silicon by acid-leaching of silicon powder as well as by unidirectional solidification of silicon. Some of these processes are less expensive than the conventional method for obtaining high purity silicon from chlorosilane reduced --pyrolyzed in H.sub.2 to produce pure polycrystalline silicon which can cost as much as 70 times the metalurgical grade silicon starting material. However, most of the other methods proposed either involve high costs or are of limited value in producing a very high purity silicon, such as needed for solar applications, i.e., a purity of 99.999 to 99.9999%.
In studying the thermodynamics of reactions between impurities in molten silicon versus the same impurities in solid silicon when contacted by certain materials, we have determined the surprising effect that reactions between impurities in solid silicon and certain materials are more favorable than reactions between the same impurities in molten silicon in contact with the same molten materials. Furthermore, we have found that the relative purification power of such materials for solid silicon with respect to liquid silicon can be even greater, if the solid silicon is crushed to expose the grain boundaries of adjacent grains in the polycrystalline silicon, apparently due to the tendency of the impurities to concentrate along the grain boundaries of adjacent crystals during solidification of the silicon.